Parametric sound is a fundamentally new class of audio that relies on a non-linear mixing of an audio signal with an ultrasonic carrier. One of the key enablers for this technology is a high-amplitude, efficient ultrasonic source, which is referred to here as an emitter or transducer. Ultrasonic emitters can be created through a variety of different fundamental mechanisms, such as piezoelectric, electrostatic, and thermoacoustic, to name a few. Electrostatic emitters are generally capacitive devices consisting of two conductive faces with an air gap, where at least one of the conductive faces has a texture that is critical to the functionality of the emitter.
Non-linear transduction results from the introduction of sufficiently intense, audio-modulated ultrasonic signals into an air column. Self-demodulation, or down-conversion, occurs along the air column resulting in the production of an audible acoustic signal. This process occurs because of the known physical principle that when two sound waves with different frequencies are radiated simultaneously in the same medium, a modulated waveform including the sum and difference of the two frequencies is produced by the non-linear (parametric) interaction of the two sound waves. When the two original sound waves are ultrasonic waves and the difference between them is selected to be an audio frequency, an audible sound can be generated by the parametric interaction.
Parametric audio reproduction systems produce sound through the heterodyning of two acoustic signals in a non-linear process that occurs in a medium such as air. The acoustic signals are typically in the ultrasound frequency range. The non-linearity of the medium results in acoustic signals produced by the medium that are the sum and difference of the acoustic signals. Thus, two ultrasound signals that are separated in frequency can result in a difference tone that is within the 60 Hz to 20,000 Hz range of human hearing.